Production of low-temperature fuel cell electrodes

ABSTRACT

A process for the production of electrodes for fuel cells includes applying an ink consisting of a mixture of at least a carbon powder, an ion-conducting polymer and a fluid, the fluid containing at least 3% of an alkanediol, and a fuel cell electrode which can be obtained by applying such an electrode ink to a substrate, followed by a heat treatment.

BACKGROUND OF THE INVENTION

The invention relates to the production of an electrode, the mostimportant components of which consist of a noble metal catalyst and aproton-conducting polymer. Such electrodes are used, inter alia, in fuelcells which contain a proton-conducting polymer membrane as electrolyte(SPFC, Solid Polymer Fuel Cell). A fuel cell of this type is able toconvert chemical energy into electrical energy and heat in a clean,quiet and efficient manner. Possible applications are, inter alia,electric transport, heat/power generation on a scale of 1–250 kW andportable equipment.

Such a fuel cell has two electrodes, an anode and a cathode, at which,respectively, a fuel is oxidised and oxidant is reduced. The fuel usedcan be hydrogen, a hydrogen-containing gas or an organic compound, forexample methanol. The oxidant used is usually atmospheric oxygen.

The optimum operating temperature of a low-temperature fuel cell basedon a proton-conducting polymer is around 60–80° C. The majority ofactive electrodes for the oxidation of hydrogen and the reduction ofoxygen at such temperatures and in an acid medium contain platinum ascatalytically active material. Hydrogen-containing gases which areproduced by the reaction of a hydrocarbon in a so-called reformer alsocontain, inter alia, carbon dioxide and carbon monoxide in addition tohydrogen. Carbon monoxide in particular has a highly adverse effect onthe activity of platinum for the electrochemical conversion of hydrogento protons. A catalyst that contains a mixture of platinum and a secondmetal, for example ruthenium or molybdenum, in general has a higheractivity for electrochemical oxidation of hydrogen in carbonmonoxide-containing gases than catalysts based on platinum. With regardto the reduction of oxygen, it is known that catalysts consisting of amixture of platinum and a second metal, for example chromium or nickel,can have a higher activity than catalysts based on platinum alone.

For efficient utilisation of expensive noble metals in fuel cellelectrodes it is extremely important that the surface area/mass ratio ofthe noble metal used is as high as possible. This is achieved byapplying the noble metal from a solution to a support material in such acontrolled manner that the crystallite diameter is approximately 2–4 nm.The support material used is generally carbon because of the requisiteelectrical conductivity. By making use of a carbon with a high surfacearea per unit mass it is possible to apply an appreciable quantity ofnoble metal per unit volume of carbon. Widely used support materials areVulcan XC-72, a carbon powder with a BET surface area of approximately250 m²/g, Shawinigan Acetylene Black, a carbon powder with a BET surfacearea of approximately 80 m²/g, and Black Pearls, a carbon powder with aBET surface area of approximately 1475 m²/g.

The requirement for a high electrochemical rate of reaction per unitcatalytic surface area is that the catalytic surface area is readilyaccessible to the gaseous reactants, and to protons and electrons. Inaddition, in the case of the oxygen reduction reaction it must bepossible efficiently to discharge the water produced in order thus tokeep the accessibility to oxygen high. For good accessibility to gaseousreactants, the electrode must have a certain porosity, which in SPFCswhich function well is of the order of 50%. To achieve a sufficientlyhigh proton conductivity use is usually made of electrodes which, inaddition to platinum on carbon, also contain the same proton-conductingpolymer as that used to produce the electrolytic membrane. Thepercentage of proton-conducting polymer must not be too high, since theelectronic conductivity and the gas accessibility decrease as thecontent of proton-conducting polymer increases. In general, aconcentration of proton-conducting polymer of 10–50%, in particular20–30%, based on dry weight, is suitable.

An SPFC electrode consists roughly of two different layers: a thincatalytic layer approximately 5–20 μm thick, where the actualelectrochemical reaction takes place, and a thick porous layerapproximately 100–300 μm thick, which is termed the electrode backing.The function of this thick layer is to distribute the gas to electrodesections which are not opposite a gas channel, to guide electrons in thelateral direction and to ensure effective water transport from and tothe electrode.

The catalytic layer can be applied either to the electrode backing or tothe electrolytic membrane. Various techniques for application are known,including atomising, screen printing and coating. In order to make useof these techniques the noble metal-containing carbon particles and theproton-conducting polymer must have been dispersed in a suitablesolvent. This dispersion is termed ink. The entire dispersion must havea rheology which makes it possible to process the ink in themanufacturing equipment used. In addition, the solvents used mustevaporate within a practical timescale. Evaporation that is too rapidleads to a changing rheology during electrode production, with theconsequence that the production of electrodes is not reproducible. Inaddition, evaporation that is too rapid leads to agglomeration of solidink components, as a result of which the production process isinterrupted. However, it must be possible to remove the solvents used ata temperature of at most 150° C. at a reasonable speed, within at mostone hour. Above this temperature of 150° C. changes take place in theproton-conducting polymer in the electrode, as a result of which protonconductivity in the electrode decreases.

In order to obtain a well-dispersed electrode ink use is often made ofadditives such as binders and surfactants. The function of a surfactantis to reduce any repulsions between the surface of the dispersedparticles and the dispersing medium so as thus to obtain a stabledispersion. A binder is in general a component that has the effect ofincreasing the viscosity.

Examples of components which have the effect of increasing the viscosityare carboxymethylcellulose, polyethelene glycol, polyvinyl alcohol,polyvinylpyrrolidone and other polymer compounds. As a consequence ofthe polymer character of such compounds which increase the viscosity,these compounds form part not only of the electrode ink but also of thefinal electrode. Not only is this component then an electrodeconstituent that has no function in the final electrode but, byinteraction with the noble metal surface of the active phase, such acomponent can also have an adverse effect on the electrochemicalactivity of the electrode. This results in a reduced capacity per unitof electrode surface.

U.S. Pat. No. 5,330,860 in the name of W. Grot et al. teaches that theproton-conducting, perfluorinated sulphonic acid polymer, such asNafion, required for the electrode can serve as binder in the electrodeink. Addition of a supplementary component that increases the viscositybecomes superfluous as a result. According to the cited patent, thesolvent used is an ether, preferably 1-methoxy-2-propanol. However, sucha solvent has too high a vapour tension at room temperature,specifically 12 mbar, as a result of which the viscosity of theelectrode ink is subject to change during the electrode productionprocess. Such an ether compound also has adverse consequences forhealth.

An attractive alternative to the use of a hydrocarbon such as1-methoxy-2-propanol is water. The use of water as solvent in anelectrode ink is described in U.S. Pat. No. 5,716,437 in the name ofDenton et al. Water has no effect whatsoever on health and, if suitable,would be the ideal solvent for the production of electrodes. However,water has too high a vapour tension at room temperature, specifically 17mbar. As a consequence the viscosity of the electrode ink changes duringthe production process. In addition it is very difficult to printhydrophobic surfaces, which include the electrode backing surfaces whichare most common for use in an SPFC, with a water-based ink.

An electrode ink which consists of two immiscible components isdescribed in EP-A 0 945 910. One of the components is an ink whichcontains the catalyst with the conducting polymer (ionomer) in a polarsolvent such as an alcohol or diol, for example propylene glycol,dipropylene glycol, glycerol or hexylene glycol. The other component isan ink containing catalyst without ionomer in an apolar solvent, such asfatty acid esters, for example methyl dodecanoate. After combining thetwo inks, an electrode having an inhomogeneous microstructure isproduced, the inhomogeneity serving to improve the gas transport in thecatalytic layer and thus to increase the capacity of the fuel cell.However, the method according to EP-A 0 945 910 is laborious and,moreover, the electrode performance is not yet completely satisfactory.

A method for electrode production in which the starting material used isa colloidal solution of the polymer is described by M. Uchida et al.,“New Preparation Method for Polymer-Electrolyte Fuel Cells”, J.Electrochem. Soc. 142 (1995), 463–468. Propanediol is regarded as anunsuitable solvent by Uchida et al. because it is not possible to formpolymer colloids therein because the dielectric constant of propanediolis too high.

SUMMARY OF THE INVENTION

The present invention solves the problems of the prior art describedabove. Surprisingly, it has been found that a homogeneous electrode canbe obtained very suitably using an electrode ink based on an alkanediol,in particular 1,2-propanediol (propylene glycol), optionally mixed withsolvents miscible therewith, which electrode can, moreover be producedmore simply and/or has a better performance than the fuel cellelectrodes known to date. In the text which follows reference is alwaysmade to 1,2-propanediol, but other alkanediols, in particular C₃–C₄alkanediols, such as 1,3-propanediol, 1,2- and 1,3-butanediol anddiethylene glycol, can also be used. The invention therefore relates toa method for the production of an electrode using an electrode ink whichcontains the customary constituents discussed above, in particular acarbon support or other suitable support with one or more catalystmetals optionally bonded thereto, and an ion-conducting polymer, thesolvent at least partially consisting of an alkanediol, preferably1,2-propanediol. The invention also relates to the electrodes and fuelcells, or capacitors, obtainable by this method. The invention isdescribed in more detail in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A current/voltage plot of the Example 2 cell is shown in FIG. 1 and thevoltage at a given current density against time in FIG. 2.

The current/voltage plot and the voltage measured as a function of timeare shown in FIGS. 3 and 4 for Example 3.

The current/voltage plot of the Example 5 cell is shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solvent of the electrode ink therefore consists to at least 3% of analkanediol. In one embodiment of the method according to the invention,the ink fluid also contains water, for example 80–96% water and 4–20%1,2-propanediol. The fluid can, however, also be a mixture ofalkanediols, or of alkanediols on the one hand and other polar solvents,such as alcohols, alcohol ethers, ethers, esters, amides or sulphoxides,on the other hand, with preferably at least 50%, in particular at least70% alkanediol. The fluid can also consist completely of alkanediol.

The very low vapour tension of 1,2-propanediol, at room temperature, of0.2 mbar ensures that no 1,2-propanediol evaporates in discernibleamounts during the use of an electrode ink containing 1,2-propanediol assolvent. The viscosity and processibility of such an ink consequentlyremains constant for a prolonged period of a few hours. In addition,this compound can be adequately removed at elevated temperature, 80–90°C., within a timescale of a few minutes, as a result of which the finalelectrode can be further processed. With a view to possible harmfulconsequences to health, 1,2-propanediol is an acceptable compound. Thetoxicity of 1,2-propanediol is low; it is also used for the preparationof foodstuffs and dermatological products. Because of its very lowvapour tension, exposure by inhalation is very easy to prevent. Finally,the polarity of the compound is such that ink based on 1,2-propanediolcan be applied easily to both hydrophobic and hydrophilic surfaces.Hydrophobic surfaces such as electrode backings can consequently beprinted without too much pressure using an electrode ink based on1,2-propanediol. In addition, the electrolytic membrane, which isusually hydrophilic, can also be printed using the electrode ink of thisinvention. In general, swelling of the membrane takes place during theapplication of electrodes to an electrolytic membrane. This swellingleads to the electrode and the electrode/membrane interface coming undertension. The adhesion of the electrode to the membrane is adverselyaffected by this swelling behaviour. In addition, small cracks arise inthe electrode itself, which has an adverse effect on the electricalcontact between the electrode particles. The degree of swelling dependson the solvent. According to the study by R. S. Yeo, published in thejournal Polymer, Vol. 21, (1980), page 433, the most important parameterdetermining this degree of swelling is the solubility parameter.According to this study the solubility parameter would have to be closeto 0 for minimum swelling, and maximum swelling occurs for solvents witha solubility parameter of 10 ((cal.cm⁻³)^(0.5)). According to this study1,2-propanediol ought to produce a swelling comparable to that ofprimary alcohols such as 1-propanol and ethanol. In practice, however,it is found that a membrane does not swell at all on absorbing1,2-propanediol, which is of extremely great importance for obtainingdimensionally stable electrodes when these are applied directly to themembrane. In this case the increase in the length of a rectangular pieceof membrane was taken as a measure of the swelling. Whilst the increasein water is 10% and that in 1-propanol 18%, the increase in1,2-propanediol is 0%.

The method according to the invention can be carried out in a mannerknown per se. The carbon powder is loaded with 5–60% (m/m), inparticular 10–45% (m/m), of at least one platinum metal, preferablyplatinum itself. A second metal such as ruthenium or molybdenum, orchromium, nickel, palladium, cobalt or iridium, can be added hereto inan amount of 0.1 to 75% (m/m), based on the total weight of catalystmetal. The requisite amount of the proton- or ion-conducting polymer, inparticular a polymer that contains perfluoralkylsulphonic acid groups(—C_(n)F_(2n)SO₃H), is added to this, for example 10–40% (m/m) based onthe carbon/catalyst mixture. Before or after the addition of the polymeror, preferably, at the same time as the polymer, the solvent is added,for example in an amount which leads to a catalyst solids content of0.1–2 g/ml, in particular 0.5–1.0 g/ml. If necessary, one or moredispersion steps are carried out. The ink in liquid or paste form isthen applied in a known manner to either an electrode backing, in alayer having a thickness of 2–50, in particular 4–30, μm, or anelectrolyte layer, after which drying is carried out at a temperature ofbetween 75 and 150° C. The electrolyte layer or the electrode backing isthen applied, usually with heating (75–150° C.) and optionally underpressure (2–100 bar). A super-capacitor electrode can be produced in acomparable manner, with the proviso that the substrate then usuallyconsists of an electron-conducting film or foam.

Comparing the performance of an inhomogeneous electrode reported in EP 0945 910 A2, i.e. 540 mV at a current density of 500 mA/cm², with theperformance of a homogeneous electrode according to the presentinvention, i.e. 600 mV at a current density of 500 mA/cm², obtainedunder identical fuel cell conditions shows that the homogeneouselectrode according to the invention is to be preferred.

The following examples illustrate the use of electrode inks based on1,2-propanediol and demonstrate that fuel cells produced using thiselectrode ink have performances comparable to those of fuel cells thathave been produced using electrode inks containing water as solvent. Thefuel cell tests were carried out under the following test conditions:

Type of fuel: hydrogen Type of oxidant: air Pressure: 1.5 or 1.0 barCurrent density: 470 mA/cm² Type of flow: counterflow Cell temperature:65° C. Wetting temperature: 65° C. Hydrogen/air reactant stochiometry:1.5/2.0

In all examples the electrode surface area tested was 7 cm². However,production was also carried out up to surface areas of 310 cm² withoutthe electrode loading becoming inhomogeneous at this scale. Theelectrode backings used were all of the ETEK-Elat double-sided orsingle-sided type. These electrode backings consist of a carbon clothwith a micro-porous, hydrophobic layer on one or two sides. This layercan be inked well without the electrode ink penetrating deeply into theentire structure. This deep penetration of the electrode ink has anadverse effect on the transport of oxygen to the electrochemicallyactive part of the fuel cell cathode. It also leads to inefficient useof the catalyst containing noble metal. This problem can, however, ariseif use is made of electrode backings based on macroporous graphitepaper, as sold by Toray. This graphite paper can be used, following astep for rendering it hydrophobic, as electrode backing in a solidpolymer fuel cell (SPFC). The problem outlined can be solved by addingwater to the electrode ink. Thus, an electrode ink in which the fluidconsists of 90% water and 10% 1,2-propanediol is exceedingly suitablefor applying electrodes to electrode backings without this inkpenetrating deeply into the electrode backing. A fluid consisting of 5%propanediol and 95% water is also found to be extremely suitable. Anafter-treatment step is required in order to obtain adhesion of thiselectrode to the hydrophobic graphite paper, which after-treatment stepessentially consists in a heat treatment at 130° C. for one hour. Toobtain optimum fuel cell performance, this electrode must be impregnatedwith a Nafion solution that predominantly consists of water.

EXAMPLE 1

An electrode ink suitable for screen printing on an electrolyticmembrane or on an electrode backing is prepared as follows. 16 g heptaneis added to an amount of 2.0 g 40% (m/m) Pt/Vulcan XC72. The whole ismixed well until a dispersed mixture forms. The heptane is evaporatedunder a gentle stream of nitrogen. An amount of 9.6 g 5% Nafionsolution, obtainable from DuPont or Solution Technology Inc., isevaporated to dryness at room temperature. 9.6 g methanol is added tothe Nafion that has been evaporated to dryness and the mixture is thentreated in an ultrasonic bath for 20 minutes, if necessary at anelevated temperature of, for example 60° C. 1 g 1,2-propanediol is thenadded. The methanol is evaporated at 60° C. in a vacuum rotaryevaporator until no further distillate is collected. The residue isdiluted with 1,2-propanediol to a final concentration of 7.5% Nafion in1,2-propanediol, which corresponds to 6.4 g solution. 2.0 g 40%Pt-on-Vulcan is added to this 6.4 g 7.5% Nafion in 1,2-propanediol. Theresulting mixture is heated at 100° C. for 2 minutes, followed by adispersing step for one minute. After cooling, for example in arefrigerator, the ink is ready for use.

EXAMPLE 2

The ink prepared in accordance with Example 1 was applied with the aidof screen printing on a DEK 247 screen printing machine to an electrodebacking, purchased from E-TEK Inc. under the name single-sidedELAT-electrode-Carbon only. After applying the electrode to theelectrode backing, the ink was dried for 3 minutes at 90° C. under inertconditions (nitrogen atmosphere). A screen printed electrode was thenapplied by means of a hot pressing step (130° C., 40 kg/cm²) to twosides of a 50 μm thick electrolytic membrane of the Aciplex-S1002 type,purchased from Asahi Chemical. The fuel cell thus obtained had aplatinum loading of approximately 0.3 mg/cm² on both electrodes. Theresulting fuel cell was tested under the conditions described above. Thecurrent/voltage plot of this cell is shown in FIG. 1 and the voltage ata given current density against time is shown in FIG. 2.

EXAMPLE 3

The ink prepared according to Example 1 was applied with the aid ofscreen printing on a DEK 247 screen printing machine to both sides,precisely opposite one another, of a proton-conducting membrane of theNafion 115 type, purchased from DuPont de Nemours Inc. After applyingthe electrode to the electrolytic membrane, the ink was dried for 3minutes at 90° C. under inert conditions (nitrogen atmosphere). Theresulting membrane/electrode combination was stored for 24 hours in 0.1M H₂SO₄ to remove 1,2-propanediol from the membrane. An electrodebacking, purchased from E-TEK Inc. under the name double-sidedELAT-electrode-Carbon only, was then applied to both sides of themembrane-electrode combination in contact with the screen-printedelectrode by means of a hot pressing step (130° C., 40 kg/cm²). The fuelcell thus obtained had a platinum loading of approximately 0.3 mg/cm² onboth electrodes.

For comparison, a fuel cell was produced in a manner identical to themethod described in Example 2, but on an electrode backing, purchasedfrom E-TEK Inc. under the name double-sided ELAT-electrode-Carbon only,and making use of a Nafion 115 membrane. Both fuel cells were testedunder the conditions described above. The current/voltage plot and thevoltage measured as a function of time are shown in FIGS. (sic) 3 and 4.[lacuna] from the comparison that both the short-term performance andthe long-term stability yield a virtually identical result for bothapplication methods.

EXAMPLE 4

An electrode ink suitable for screen printing on an electrolyticmembrane or on an electrode backing is prepared as follows: an amount of8.8 g 1,2-propanediol is added to an amount of 2.0 g 30% (m/m) Pt/VulcanXC72. A (sic) 2.0 g of a solution of 25% (m/m) Nafion in 1,2-propanediolis added to this mixture and the whole is dispersed until a thick pastehas formed.

EXAMPLE 5

The ink prepared according to Example 4 was applied with the aid ofscreen printing on a DEK 247 screen printing machine to an electrodebacking consisting of graphite paper of the Toray make, provided with acarbon layer of a thickness of approximately 5 μm and renderedhydrophobic. After applying the electrode to this electrode backing, theink was dried for 3 minutes at 90° C. under inert conditions (nitrogenatmosphere). A screen-printed electrode was then applied to two sides ofa 50 μm thick electrolytic membrane, of the Nafion 112 type purchasedfrom DuPont, by means of a hot pressing step (130° C., 40 kg/cm²). Thefuel cell thus obtained had a platinum loading of approximately 0.22mg/cm² on both electrodes. The resulting fuel cell was tested under theconditions described above, both under 1.5 bar and under 1 bar. Thecurrent/voltage plot of this cell under both conditions is shown in FIG.5.

1. A process for producing an electrode, comprising the steps of:bonding carbon particles with catalytically active metals to form ametal-bonded carbon powder; applying an ink which contains at least thecarbon powder, a proton-conducting polymer and one or more misciblepolar solvents to an electron-conducting or ion-conducting substrate,wherein, the ink is homogeneous and only contains solvents that arepolar and that are miscible with each other, the solvents comprise atleast 3% by weight of an alkanediol having 3–6 carbon atoms, and theproton-conducting polymer is in its acid form.
 2. The process accordingto claim 1, wherein the alkanediol is 1,2-propanediol.
 3. The processaccording to claim 1, wherein the proton-conducting polymer is aperfluorinated sulfonic acid.
 4. The process of claim 3, wherein theproton-conducting polymer is a copolymer of tetrafluorethylene andperfluorosulfoethyl vinyl ether.
 5. The process according to claim 1,wherein the catalyst contains a noble metal or an alloy of a noble metalwith a second metal and optionally a third metal.
 6. An electrodeobtained by the steps of: providing one of an electron-conductingsubstrate, and an ion-conducting substrate; applying, to the providedsubstrate, an homogeneous ink comprising a carbon bonded withcatalytically active metal powder, a proton-conducting polymer in itsacid form, and one or more miscible polar solvents comprising at least3% by weight of an alkanediol having 3–6 carbon atoms, the homogeneousink containing only polar solvents.
 7. The electrode according to claim6, wherein the substrate comprises an electrode backing.
 8. Theelectrode according to claim 6, wherein the substrate comprises anion-conducting membrane.
 9. The electrode according to claim 8, whereinthe ion-conducting membrane is proton-conducting.
 10. A fuel cellcontaining electrodes according to claim
 6. 11. A process of producingan electrode, comprising the sequential steps of: preparing a carbonsupport powder by bonding carbon particles with one or more catalystmetals; preparing a homogeneous ink comprising the carbon supportpowder, a proton-conducting polymer in its acid form, and one or moremiscible polar solvents comprising at least 3% by weight of analkanediol having 3–6 carbon atoms; and applying the homogeneous ink toa substrate, the homogeneous ink containing only polar solvents.
 12. Theprocess of claim 11, wherein, the substrate is one of anelectron-conducting substrate and an ion-conducting substrate.
 13. Theprocess of claim 11, wherein, the substrate is one of anelectron-conducting film and electron-conducting foam.
 14. The processof claim 11, wherein, a proton-conducting polymer used is aproton-conducting polymer in acid form.
 15. The process of claim 11,wherein, a proton-conducting polymer used is a proton-conducting polymerin acid H⁺ form.
 16. The process of claim 11, wherein the ink contains0.1–2 g of the content of metal-bonded carbon support powder per ml. 17.The process of claim 11, wherein, in preparing the homogeneous ink, thepolar solvents comprises at least 50% by weight of the alkanediol having3–6 carbon atoms.
 18. The process of claim 11, wherein, in preparing thehomogeneous ink, the solvent comprises 4–20% by weight of the alkanediolhaving 3–6 carbon atoms and 80–96% by weight of water.